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Symptoms vary in relation to moisture and temperature levels. Onion smudge may apppear at all growth stages but it is more common on bulbs approaching maturity and during storage.
The disease affects the scales and lower portions of the unthickened leaves, which constitute the neck of the bulb.
On the outer bulb scales, over the surface of the bulb, masses of minute stromata form beneath the cuticle. These bodies, dark green at first, blackening with age, are sometimes scattered over the surface of the bulb but are more frequently concentrated in smudge-like spots, roughly circular, or arranged in concentric rings 1-2 cm diameter or more. Under moist conditions, the stromata produce spore-bearing structures (conidiomata) with cream-coloured spore masses and typical black, stiff, bristles (setae), readily distinguishable with a hand lens.
In most cases the infection can remain confined to one or two of the outer dry scales. In contrast, under favourable conditions of high moisture and temperature, which occur during harvesting in wet weather and storage under wet conditions, the fungus spreads and grows into underlying scales where it produces similar spots surrounded by yellowish borders. In severe cases, infection can penetrate the entire bulb causing shrinkage and complete collapse of the fleshy scales. The disease increases onion susceptibility to other rotting agents.
C. circinans may directly attack the inner fleshy scales exposed to soil by cracking the outer scales. The symptoms consist of minute, sunken, yellowish spots that can enlarge and join together.
Coloured onions with few outer scales can also be affected by smudge (Walker, 1921; Dingley, 1961; Agosteo and Polizzi, 1997). Before the fungus penetrates the cell lumen, the pigment confined to the outer epidermal cells of fleshy scales disappears and a typical unpigmented halo is formed around the smudge (Agosteo and Polizzi, 1997). On these tissues the fungus can differentiate into stromata and conidiomata.
In seedlings, smudge is very serious, causing damping-off and total loss of seedlings. On coloured varieties, infection takes place before the appearance of pigments in the outer scales and on the unpigmented portions of bulbs, i.e., at the top and near the neck where there are few or no phenolic compounds (Walker, 1921, 1923; Link et al., 1929b) (see Biology and Ecology).
Smudge may cause a reduction in weight and sprouting of onion sets, with heavy damage especially where the cultural cycle involves a long period of storage that expose sets to smudge infection for several months (Walker, 1921). Sets that sprout during storage must be discarded.
Another symptom, especially seen in warm and wet climates, is a leaf anthracnose with elliptical, greenish or yellowish-grey, later brown, spots with yellow halos which can result in a leaf blight.
Cultural Control and Sanitary Methods
Where smudge prevails on white cultivars and coloured cultivars with few (one or two), easily-sloughing, outer scales, a 2-3 year crop rotation is suggested. Good drainage of soil is another important agronomic practice especially in onion-growing areas.
On white varieties it is necessary to avoid harvesting during wet weather, to remove all affected bulbs at digging and to dry bulbs before storage and marketing. This purpose is more easily achievable with natural drying, placing bulbs under sunlight for a few days. Placing bulbs in a dry and windy place before storage is also advisable. If wet weather occurs during harvesting it is advisable to dry the outer scales of bulbs in tunnels or chambers with forced circulation of warm air. It is necessary to avoid wet conditions during storage, with temperatures of <20°C.
Use of healthy transplants is very important. Cultivation on soil which has not produced diseased crops in the previous 2-3 years is recommended. For transplants more elaborate curing procedures are justified to reduce post-harvest disease development (Walker, 1969) including forced circulation of warm air (up to 48°C) until the outer scales are completely dry and storage at just above 0°C and 65% RH.
The best means of controlling this disease is the cultivation, where feasible, of coloured varieties with well-developed outer scales (see Biology and Ecology).
Wietsma and De Vries (1990) advocate the possibility of incorporating C. circinans resistance genes in leek by interspecific breeding from wild relatives, including A. ampeloprasum. The latter species is regularly used in interspecific breeding programmes to transfer disease resistance genes to onion (Van der Meer, 1988).
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
The commonest smudge symptom in most world cultivation areas is a light infection of outer scales on mature white onions, shallots and leeks. The disease subsequently causes a reduction of bulb market value as a result of the marred appearance.
Some estimates of disease losses have been done in the past. Thaxter (1890) referred to heavy economic damage in Connecticut, USA. If white varieties are grown in infested areas or the bulbs are harvested in wet weather and are not dried properly before storage and bagging, serious losses may result. Under prolonged storage, smudge may cause complete collapse of fleshy scales, shrinkage and premature sprouting of the bulbs and increased susceptibility to various other rotting agents.
In southern Italy, Agosteo and Polizzi (1997) report a 30% disease incidence on red onions harvested at the initial bulb enlargement stage. They also refer to the greater time and costs required for product cleaning in storage and bagging.
Leaf anthracnose and leaf blight have been reported in Italy, Puerto Rico (Sumner, 1995), Japan (Kondo and Hemmi, 1953), China (Shi and Tong, 1997), India (Narain and Saksena, 1971) and England, UK (Salmon and Ware, 1935). Shi and Tong (1997) reported serious yield losses (20-50%) on scallion in China.
Serious infection of seedlings with damping-off and total losses are reported in the USA (Walker, 1921; 1969), Germany (Behr, 1963), Spain (Sagasta Azpeitia, 1969) and Mauritius (Orian, 1953). In Italy, Togliani (1951) referred to the occasional damping-off of seedlings caused by C. circinans (see Biology and Ecology).
Walker (1921) refers to heavy economic damage as a consequence of shrinkage and sprouting of onion sets in Chicago, USA.